Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically comprise thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's).
For many years in the semiconductor industry, optical lithography techniques such as contact printing, proximity printing, and projection printing have been used to pattern material layers of integrated circuits. Projection printing is commonly used in the semiconductor industry using wavelengths of 248 nm or 193 nm, as examples. At such wavelengths, lens projection systems and transmission lithography masks are used for patterning, wherein light is passed through the lithography mask to impinge upon a wafer.
However, as the minimum feature sizes of IC's are decreased, the semiconductor industry is trending towards the use of very short wavelength, immersion lithography technologies or non-optical lithographic techniques to achieve the decreased feature sizes demanded by the industry.
For lithographic printing of integrated circuit patterns below about 50 nm feature sizes, EUVL technology using light in the soft x-ray range (e.g., about 10 to 15 nm) is under development. Ultraviolet (UV) light has a shorter wavelength than visible light. For example, UV light is usually considered to fall within the wavelength range of about 157 to 400 nm. In EUVL, extreme UV (EUV) light, having a shorter wavelength than UV light, e.g., about 13.5 nm, is used as the wavelength. In EUVL, plasma is used to generate a broadband radiation with significant EUV radiation. The EUV radiation is collected by a system of mirrors coated with EUV reflecting interference films, also known as Bragg reflectors. The EUV radiation is then used to illuminate an EUV reflection lithography mask. The pattern on the lithography mask is imaged and de-magnified onto a resist-coated wafer. The entire lithography mask pattern is exposed onto the wafer by synchronously scanning the lithography mask and the wafer.
EUV radiation or EUV light used in EUV lithography (EUVL) systems may be generated by heating fuel materials such as xenon, lithium, or tin via discharge produce plasmas (DPP) or via laser produced plasmas (LPP). In DPP EUV sources, fuel is heated via magnetic compression, while in LPP, fuel is bombarded with a focused laser beam to produce the heating. However, in the process of generating EUV radiation, in these methods, additional unwanted radiation and debris are produced. The debris includes particle debris, e.g., micron sized particles, and high energetic atomic species. This debris can include particles (neutral or charged), ions, neutral atoms, molecules (neutral and ions), and electrons, as examples.
In an EUV lithography system, in order to extract the EUV light, an optic mirror is used. The optic mirror is costly, has a certain useful lifetime, and cannot be replaced frequently, and thus needs to be protected from the debris. The optic mirror needs to be protected such that as much of the EUV light passes through as possible, while the debris is prevented from reaching the optic mirror. The kinetic energy of the debris can be very high and may cause erosion of the EUV system collecting optics, such as a grazing incidence collector, multilayer mirrors, or near normal incident Bragg reflector mirrors used in the sources as EUV light collectors. The erosion of the EUV system collecting optics is caused by kinetic energy sputtering of the debris produced by the generation of the EUV radiation, while there may be other contributing factors to this erosion, such as potential sputtering and chemical erosion, as examples.
In addition, some of the debris comprises highly charged ions that are generated by the EUV producing plasma. The highly charged ion debris can be very damaging to EUV system collecting optics surfaces, even at very low kinetic energies. This erosion is caused by potential energy sputtering of the highly charged ion debris. The damage caused by this kind of debris to the EUV collecting optics in EUV sources significantly reduces the lifetime of the EUV lenses and mirrors, and increases cost of ownership (COO) for EUV sources and EUV lithography systems.
Mitigating debris to prevent the debris from reaching the collecting optics is considered one of the largest challenges in the development of EUV lithography. Extending the lifetime of the plasma-facing collector mirror, which delivers the EUV light into focus, is one of the most critical issues for EUV lithography development, for example. “Foil trap” based debris mitigation devices are generally used today by the industry to mitigate the debris, i.e., to prevent it from reaching EUV mirror surfaces, as described by Shmaenok, L. A., et al., in “Demonstration of a Foil Trap Technique to Eliminate Laser Plasma Atomic Debris and Small Particulates,” Proceedings of SPIE, 1998, pp. 90-94, Vol. 3331, which is incorporated herein by reference, and also in U.S. Pat. No. 6,838,684, issued on Jan. 4, 2005 to Bakker et al., which is also incorporated herein by reference. The foil trap devices trap debris on a system of foils near the EUV light source, or divert particles by bouncing them off surfaces. However, there are limitations of using foil trap based debris mitigation devices. The transmission loss of such devices is typically about 40 to 60% of the EUV light generated by the plasma. Furthermore, a significantly large amount of the debris passes through the foil trap debris mitigation devices, causing erosion and damage, and debris build-up on the collecting optics for EUV light.
Other debris mitigation devices and methods include repeller fields, in which an electric field is used to repel ions or charged particles, as described by Takenoshita, K., et al., in “The Repeller Field Debris Mitigation Approach for EUV Sources,” Proceedings of SPIE, 2003, pp. 792-799, Vol. 5037, which is incorporated herein by reference, and also in U.S. Pat. No. 6,377,651 issued on Apr. 23, 2002 to Richardson et al. on Apr. 23, 2002, and U.S. Pat. No. 6,614,505 issued on Sep. 2, 2003 to Koster et al., which are incorporated herein by reference. Uncharged particles are, however, allowed to pass through repeller fields. Other debris mitigation attempts include using metal meshes coated with a material; background gases like Krypton, Argon, or Helium; Cu or Ta tape; and RF plasma in combination with a foil trap, as examples. While these methods and devices increase optics protection, these methods do not completely eliminate the debris emanating from EUV light sources that causes damage to and built-up on the collecting optics and other components of the EUV lithography system. Also, these prior art methods and devices significantly reduce the EUV light transmission, thereby increasing the time it takes to expose a wafer and thus significantly increasing cost of ownership for EUV lithography systems.
Furthermore, the lifetime of existing EUV light collection optics in the EUV lithography systems is about 500 million pulses, due to the large amount of debris that causes damage to the optical components. A lifetime of about 50 to 100 billion pulses is needed in the art, for EUV lithography to be a viable and cost-effective lithography method in high volume production.
Thus, what are needed in the art are improved devices and methods of filtering debris that do not excessively lower the EUV light transmission in EUV lithography systems.